This invention relates to a method of and apparatus for sequentially continuous casting different compositional grades of steel. Particularly, the invention relates to a method of using a mechanical separator having particular configurations to sequentially continuously cast different compositional grades of steel in shorter times and with better yields.
Since the advent of continuous casting, numerous attempts have been made to continuously cast in sequence steels having different compositions. Continuous casting of slabs is a preferred way of making and casting steel because of the lower production costs associated with such casting techniques. Further advantages associated with such lower production costs could be realized if steel alloys of different compositions could be sequentially cast with a minimal time delay between casting of different steels and with minimal associated problems.
Numerous attempts by others have been made to insert mechanical separators between different compositional grades of steel slabs during casting. Such separators have the purpose of solidifying the molten steel of a first slab of one composition in the mold, as well as facilitating connecting the first slab to a second slab of a different composition. For example, it is known to use single or multi-piece structures as separators which extend substantially across the length and width of the opening of the continuous casting mold. Such structures may take the form of partition plates or grids. For example, U.S. Pat. No. 4,250,945, issued Feb. 17, 1981, discloses immersing into the molten metal, a steel structure slightly smaller than the inner cross-sectional area of the mold. The particular structure disclosed is grid-like, having a plurality of vacant spaces of various shapes for allowing the molten steel to pass vertically through the structure. It is also known to use angle-shaped devices which are immersed with the angle edge downwardly into the molten metal (i.e., the angle opened upwardly) and which may extend partially or substantially across the continuous casting mold opening. Such prior art devices generally require special placement and orientation in order to function properly. While such mechanical separators have been somewhat successful, there are numerous problems associated with the method and apparatus set forth in the prior art which have not permitted sequence continuous casting of different compositional grades of steel to be a readily acceptable and efficient practice.
One problem associated with such prior art casting is that between a first slab of one steel composition and a second slab of a different composition, there is a transition zone in which there is mixing of the two steels. Such areas of the slab generally do not have adequate or desirable properties and must subsequently be cut out of the slabs during processing. Larger zones result in lower production yields. Such a problem is further aggravated when the two steels being cast have widely different compositions.
Another problem associated with sequence casting is the inability to maintain an adequate bond or connection between the two slabs of different compositions. Still another problem is that during the process of continuous casting the different compositional grades in sequence, there may result a "breakout" in the area in and about the transition zone due to insufficient solidification such that molten metal would break through the solidifying outer shell of the slab. When breakouts occur, then there are larger yield losses associated with casting production. Still another problem associated with sequence casting is the need to solidify the end of the first slab before or while commencing pouring and casting of the second slab of steel of a different composition and the time it takes to do so. In many prior art practices, it is necessary to move equipment, such as the nozzle or casting tundish out of the way, before the mechanical separators can be inserted into place and thereafter moving the nozzle or tundish back into place before casting the steel having a different composition. Such practices are inefficient and result in further production costs.
What is needed is a method and apparatus for sequentially continuously casting steels of different compositional grades which overcome the problems of the prior art. Sequence casting should minimize the area of mixing of the steels of different compositional grades so as to improve the yield. The method and apparatus should provide for a relatively easy way of sequentially casting so that the mechanical separator may be easily immersed into the molten steel without the problems associated with the orientation of the separator, or the movement of related casting equipment. And still further, it is desirable that a method and apparatus provide for minimal stoppage during casting while yet maintaining the integrity of the slabs sequentially continuously cast with different compositional grades.